The present invention relates to a vacuum pump which outputs an atmospheric pressure in its exhaust duct and, more particularly, to a dry turbo vacuum pump of the type used for manufacturing semiconductors, food and chemicals and suited for establishing a vacuum which is free from contaminants such as oil.
In a prior art dry turbo vacuum pump which generates an atmospheric pressure in its exhaust duct, such as disclosed in Japanese Patent Publication No. 3-7039/1991, the peripheral flow pump stages are cast to very close tolerances because they are difficult to machine, and are fastened to machined centrifugal pump stages by means of screws to construct the impeller stage. It is detailed in Japanese Patent Laid-Open No. 63-61798/1988 to manufacture such impeller by a high precision casting process.
As disclosed in Japanese Patent Laid-Open No. 62-258186/1987, on the other hand, the stator of the vacuum pump is recessed in the portion of the interstage walls of the stator at the opposite side of the impeller.
In the vacuum pump according to Japanese Patent Publication No. 3-7039/1991, special assembly steps are necessary to attach the centrifugal pump stages to the peripheral flow pump stages, and also to retain the concentricity of the impeller and prevent frictional facial contact. As a result of combining multiple parts, the dimensional errors of each are accumulated, making it difficult to manage and retain the proper clearances between the impeller and the stator, so as to retain the performance required of the vacuum pump.
The casting process disclosed in Japanese Patent Laid-Open No. 3-61798/1991 for manufacturing the impeller, on the other hand, diminishes the dimensional accuracy, and causes variation in the performance of the product. Moreover, it is difficult to correct internal defects, and thus parts with defects which are discovered after the casting work must be disposed of. As a result, the production yield is not satisfactory.
Furthermore, the impeller casting process according to Japanese Patent Laid-Open No. 62-258186/1987, requires resort to the lost wax process, in which the required material is very expensive.